Is 1 object = 1 mesh exported?
The answer depends upon how the item is designed. One object can be one mesh, but one object can also contain two or more meshes. In Blender, the answer depends upon exactly how the new cube is added. If it is added in object mode, the new cube is a separate object. If the new cube, though, is added in edit mode, it may be part of the original object, even though it is a separate mesh.

The answer also might depend upon what you are importing the object into after export. Without knowing what you are going to be importing the exported object into, one cannot say for certain. It might be that the two meshes will be imported as separate objects, or as a single one.